The present disclosure relates generally to techniques for determining an accurate depth of an earth formation penetrated by a drill bit of a drilling system.
Wellbores are drilled in the earth, for among other purposes, to locate and produce hydrocarbons. Wellbore drilling includes the use of a drilling tool with a bit at one end that is advanced into the ground using a pipe called a “drill string” to form a wellbore. The drill string and the drilling tool are typically made of a series of tubular drill pipe sections that are connected together, such as by threaded connections, to form the drill string with the bit at the lower end thereof. In most drilling operations, as the drilling tool is advanced, drilling fluid (also called “drilling mud”) is pumped through the drill string and the drilling tool and out through the drill bit to cool the drill bit and carry away drill cuttings. The drilling mud exits the drill bit and flows back up to the surface for cleaning and recirculation through the drill string. The drilling mud is also used to form a mud cake to line the wellbore.
A bottom hole assembly (BHA) may also be located in the drill string, typically proximate the drill bit. The BHA may contain one or more measurement-while-drilling (“MWD”, for obtaining information/measurements about the drill string and/or the drill bit) and/or logging-while-drilling tools (“LWD”, for obtaining data about the earth formation penetrated by the wellbore). Additionally, the BHA may include a power generation device (e.g., a mud turbine powered by the flow of drilling mud) and a rotary steering system for controlling the direction of the drill bit to drill the wellbore along a selected trajectory.
During the drilling operation, it is desirable to provide communication between the surface and the MWD and/or LWD tools. Wellbore telemetry devices are typically used to allow, for example, power, command and/or communication signals to pass between a surface unit and the MWD and/or LWD tools and/or rotary steerable system. These signals are used to control and/or power the operation of the MWD and/or LWD tools and to send information acquired in the wellbore, which may include information obtained by the MWD and/or LWD tools in the BHA.
Various wellbore telemetry systems may be used for the desired communication capabilities. Examples of such systems may include a wired drill pipe wellbore telemetry system as described U.S. Pat. No. 6,641,434, an electromagnetic wellbore telemetry system as described in U.S. Pat. No. 5,642,051, an acoustic wellbore telemetry system as described in International Patent Application Publication No. WO2004/085796. Other data communication devices, such as transceivers coupled to sensors, may also be used to transmit power and/or data between the surface and the above described devices in the BHA.
With wired drill pipe telemetry systems, the drill pipes that form the drill string are provided with special wired pipe joints. The wired drill pipe also have one or more repeaters that contain electronics to boost the signal transmitted through the wired drill pipe between, for example, a wellbore deployed tool and a surface unit. As shown, for example, in U.S. Pat. No. 6,641,434, such wired drill pipe telemetry systems can be provided with wires and inductive couplings that form a communication chain that extends through the drill string. The wired drill pipe is then operatively connected to the wellbore deployed tool and a surface unit for communication therewith. The wired drill pipe system is adapted to pass data received from components in the wellbore deployed tool to the surface unit and commands generated by the surface unit to the wellbore deployed tool. The repeaters can be placed at predetermined intervals along the drill pipe depending on the amount the signal needs to be boosted.
The length of the drill pipe joints and repeaters are measured at the surface at atmospheric conditions and this provides the depth reference for any measurements obtained from the BHA and the repeaters. When the drill pipe is in the well and suspended by a rig, the drill pipe is under tension due to the weight of the pipe. This creates an elongation of the drill pipe as compared to its length measured at the surface. Other environmental factors may increase or decrease the length of the pipe. These factors include temperature, internal verses external fluid pressures, accumulated turns due to torque, and amount of sliding and rotating friction forces acting on the pipe. Due to this elongation and distortion, it can be difficult to determine the actual depth of a formation penetrated by the drill bit.